1. Field of the Invention
The present invention relates to a circuit substrate equipped with an insulating board formed by laminating plural insulating layers. More specifically, the invention relates to a circuit board which has functional devices with conductor portions formed in the insulating board, and is capable of decreasing the insertion losses for high-frequency signals.
2. Description of the Prior Art
A variety of electronic devices as represented by cellular phones are incorporating a circuit substrate which has functional devices with conductor portions, such as resonators, capacitors, inductors, etc. formed in an insulating board.
The circuit substrate incorporated in the electronic devices such as cellular phones is used for transmitting high-frequency signals of chiefly in the microwave band, and must have small insertion losses for the high-frequency signals. To realize low insertion losses, it is necessary to accomplish an optimum circuit design, such as matching the impedance of the circuit, suppressing undesired interference among the circuits, and improving the Q-values of the resonators. It has further been attempted to lower the dielectric loss of a dielectric material constituting the insulating board and to decrease the conduction loss of the conductor portion by forming the conductor portions by using Ag or Cu having a small volume resistivity. For example, the conductor portions having a small conduction loss can be formed in the insulating board by using a conductor such as Ag or Cu, i.e., by using ceramics (inclusive of glass ceramics), and firing the ceramics at a low temperature at which Au or Cu particles are sintered. In this case, the diameters and shapes of Ag or Cu particles are contrived, or suitable additives are added to the conductor such as Ag or Cu, so that the Ag or Cu particles are sintered to meet the behavior of sintering the ceramics, or to improve the adhesion of the conductor portions formed of Ag or Cu to the ceramics.
In modern electronic devices such as cellular phones, however, it has been highly urged to decrease the size and to improve performance. In particular, the requirement for lowering the insertion losses for high-frequency signals cannot be satisfied by the above-mentioned methods only.
It is therefore an object of the present invention to provide a circuit substrate having functional devices formed in an insulating board, and exhibits small insertion losses for high-frequency signals in the microwave band.
The present inventors have forwarded the study in an effort to decrease the insertion losses of the circuit substrate incorporating functional devices, have discovered the fact that the insertion losses can be decreased by controlling the interfacial state between dielectric ceramics constituting the insulating board and conductor portions constituting the functional devices, and by controlling the interfacial electric conductivities "sgr" of the conductor portions to be not larger than a predetermined value.
According to the present invention, there is provided a circuit substrate equipped with an insulating board formed by laminating plural insulating layers and having a functional device with a conductor portion formed in said insulating board, wherein an interfacial electric conductivity "sgr" of said conductor portion at 2 GHz is not smaller than 2.9xc3x97107 xcexa9xe2x88x921xc2x7mxe2x88x921.
In the circuit substrate of the present invention, the conductor portion which constitutes the functional device such as a resonator, a capacitor or an inductor has an interfacial electric conductivity "sgr" that is set to lie in a very high range as described above and, hence, makes it possible to decrease the insertion losses in the functional device at a high-frequency region of not lower than 800 MHz. It is therefore allowed to enhance the performance of the electronic device incorporating the above circuit substrate while decreasing noise. It is further allowed to effectively avoid an increase in the insertion losses despite of decreasing the size of the circuit substrate (or functional devices). Accordingly, the circuit substrate of the present invention is advantageous for decreasing the size of the electronic devices.
The interfacial electric conductivity "sgr" of the conducting portion vary depending upon the firing condition (particularly, depending upon the firing temperature). When the circuit substrate is produced by, for example, the cofiring, diffusion takes place excessively between the conducting material forming the conductor portion and the ceramics as the firing temperature increases, and the interfacial electric conductivity "sgr" of the formed conductor portion tends to decrease. In order to set the interfacial electric conductivity of the conductor portion to be large values as described above, it is desired that the firing is carried out at a temperature as low as possible.
Further, the interfacial electric conductivity "sgr" of the conductor portion in the microwave band vary depending greatly upon the coarseness of the surface of the conductor portion contacting to the insulating layer; i.e., the interfacial electric conductivity "sgr" increases as the surface become smooth. In the present invention, therefore, it is desired that the surface of the conductor portion contacting to the insulating layer has a coarseness Ra (JIS B 0601) of not larger than 1 xcexcm and, particularly, not larger than 0.5 xcexcm.
In the present invention, it is desired that the conductor portion constituting the functional device is formed by using Ag or Cu as a conducting material since it has a small conduction loss.
In the present invention, further, it is desired that at least one layer among the plural insulating layers is formed of dielectric ceramics containing, as a chief component, a composite oxide of Mg and Ti (e.g., MgTiO3). The insulating layer formed of such dielectric ceramics has a small dielectric loss and a high Q-value (quality value), and makes it possible to further decrease the insertion losses of the circuit substrate.
According to the present invention, further, it is desired that the neighboring insulating layers (first insulating layer and second insulating layer) are formed of dielectric ceramics of different compositions among the plural insulating layers, and that these dielectric ceramics include a common crystal phase expressed by the following formula,
(Mg, Ti)2(BO3)O
That is, in a circuit substrate incorporating the functional device, some of the plural insulating layers are often formed of insulating materials of different compositions. For example, in a circuit substrate containing a capacitor as the functional device, the insulating layer sandwitched by the conductor portions is formed of an insulating material having a high dielectric constant. In the circuit substrate in which are existing the insulating layers formed of different kinds of materials, cracks and delamination take place among the insulating layers formed of different materials and other neighboring insulating layers and the insulating board is warped due to different heat characteristics such as coefficient of contraction by firing and coefficient of thermal expansion. Upon having the above-mentioned common crystal phase existing in the first and second insulating layers that are neighboring to each other and are formed of different materials, it is allowed to bring the heat characteristics such as coefficient of contraction by firing and coefficient of thermal expansion into match together to effectively avoid the above-mentioned inconvenience. That is, the above circuit board offers an advantage of a very high degree of freedom for designing the circuit.